The DC-CDMA is a scheme for carrying out communications using a single frequency band shared by multiple users, and spreading codes are employed to identify the individual users. Here, orthogonal codes such as Gold codes are used as the spreading codes for the users.
In the process of despreading in the receiver, the interference signal power from other users are reduced by a factor of average processing gain (PG). In a mobile communications environment (particularly, under reverse link asynchronous environment), the received signals of the users undergo instantaneous fluctuations due to independent fading, short term fluctuations and distance fluctuations. Accordingly, to satisfy the intended reception quality of the users, it is necessary to carry out the transmission power control to keep constant the SIR at the input of the receiver in the base station, the SIR being defined as the ratio of the received signal power of each user to interference signal power from the other simultaneous users.
However, even if the transmission power control is perfect, and hence the SIR at the receiver input is kept constant without fail, the spreading codes are never completely orthogonal under the multipath environment in the mobile communications, and the interference is unavoidable due to cross-correlation having average power reduced by a factor of processing gain per user.
Thus, the interference signal power increases with the number of simultaneous users in the same frequency band. As a result, the capacity in terms of the number of simultaneous users per cell is determined by the receiving quality which in turn is determined by the required quality of the system.
To increase the capacity in terms of the number of simultaneous users, the cross-correlation due to other users must be reduced.
Interference cancellation techniques are proposed as one of the methods to reduce the cross-correlation from other users. As the interference cancellation technique are known: 1. a multi-user detector that demodulates not only the desired signal of the intended channel, but also the signals of other simultaneous users received at the receiver input using the spreading code information of the other users; and 2. a single user detector that minimizes average cross-correlation and noise components from other simultaneous users using the spreading code of only the intended channel. Among these, the single user detector of item 2 corrects a spreading replica code such that the cross-correlation from other users produced in the process of despreading the desired user signal is reduced through quadrature filters in the receiver.
As another technique to reduce the cross-correlation from other users to increase the capacity in terms of the number of simultaneous users, an adaptive diversity technique as shown in FIG. 1 is known. In FIG. 1, reference numerals 101A-101D each designate an antenna, 102A-102D each designate an RF stage, 103A-103D each designate an A/D converter, 104A-104D each designate a weighted coefficient multiplier, 105 designates an adder, 106 designates a demodulator, 107 designates a recovered data output terminal, 108 designates a weighted coefficient controller, and 110 designates a reference signal.
The conventional example as shown in FIG. 1 reduces the interference power from other users by attaching appropriate weights (WA-WD) to input signals to the antennas 101A-101D, and then combining them.
As another adaptive diversity technique in the DS-CDMA scheme, a method is known in which received signals input to antennas are despread before they are multiplied by appropriate weighted coefficients to be combined.
In this case, the weighted coefficients to be multiplied are successively updated so that the received SIR becomes maximum. This update enables the weighted coefficients to finally converge to such a value that will increase the gain in the direction of the incoming waves from a mobile station, but reduce the gain in the direction of incoming interference waves.
This is equivalent to providing the antennas with adaptive directivity by controlling the values of the weighted coefficients.
The adaptive control, however, is performed on the despread signals. Accordingly, it is necessary to establish the spreading code synchronization before starting the adaptive control at a base station.
In addition, a period of time required for the weighted coefficients to converge to the values that maximize the received SIR varies depending on values set as initial values of the weighted coefficients to be multiplied by the despread signals.
Furthermore, the conventional technique does not clearly disclose the procedures from establishing the spreading code synchronization based on the signal sent from the mobile station to setting the initial values of the weighted coefficients, which procedures are performed by the base station carrying out the adaptive diversity reception of the signals after despreading.